Hydrostatic compression measurements on Fe0.95O w¿stite up to 12 GPa yield a room temperature value for the isothermal bulk modulus of K0T =157 (¿10) GPa at zero pressure. This result is in accord with previous hydrostatic and nonhydrostatic measurements of K0T for w¿stites of composition: 0.89≤Fe/O≤0.95. For comparison, dynamic measurements of the bulk modulus by ultrasonic, shock wave and neutron-scattering experiments have yielded values of K0T between 140 and 185 GPa. Both high-precision compression data and simple models of the effects of defects on elastic constants indicate that the bulk modulus of w¿stite depends little on composition. Systematics among the bulk moduli of transition metal monoxides can be understood in terms of a crystal field model, although crystal field contributions to the individual elastic moduli of w¿stite are relatively small.